"We, along with the partners who reported the site, now believe it was built by a third party as part of a simulated phishing test on VoteBuilder," Bob Lord, chief security officer for the DNC, said. "The test, which mimicked several attributes of actual attacks on the Democratic party's voter file, was not authorized by the DNC, VoteBuilder nor any of our vendors."

The DNC has worked to improve its cyber defenses since it was targeted during the 2016 presidential campaign. Special Counsel Robert Mueller's Russia investigation led to indictments of 12 Russian officers in the attacks.

"The party took the necessary precautions to ensure that sensitive data critical to candidates and state parties across the country was not compromised," Lord added. "There are constant attempts to hack the DNC and our Democratic infrastructure, and while we are extremely relieved that this wasn't an attempted intrusion by a foreign adversary, this incident is further proof that we need to continue to be vigilant in light of potential attacks."